The present invention relates to a hybrid braking system for use in wheeled vehicles, such as tractor and trailers with conventional air brake, for example.
Commercial heavy duty vehicles utilize foundation brake mechanisms that are typically an S-cam design utilizing a brake drum and shoes or an air disc brake utilizing a rotor and brake pads. The foundation brake creates friction which converts the kinetic energy to heat energy thus decelerating and/or bringing the vehicle or combination vehicles to a stop. The braking systems used to activate the foundation brake mechanisms are designed to conform to one or more safety standards. Most significant is Federal Motor Vehicle Safety Standard FMVSS 121 which is the standard for Air Brake Systems—Trucks, Buses, and Trailers. This standard specifies performance, equipment and dynamometer test requirements for braking systems on vehicles equipped with air and air-over-hydraulic brake systems, to ensure safe performance under normal and emergency conditions.
The braking process is repeated each time the vehicle is brought to a stop. Effectively, all available kinetic energy produced originally from the prime mover source (most commonly a diesel or gasoline engine) is dissipated as heat and wear of friction materials.
More recently, hybrid drive systems (commonly referred to as hybrids, hybrid drives, hybrid propulsion, etc.) have been introduced. These hybrid drive systems consist of electric machines (motor, generator or combination of both) or hydraulic machines (motor, pump or combination of both) to recover energy during braking and return it to the wheels during launch or propulsion.
While this technology has demonstrated improvements in fuel economy, it is limited to drive axles because the recovered energy is only used for launch and/or propulsion. The energy from non-drive axles, including steerable axles and all axles of towed vehicles, is lost to the foundation brakes on those respective axles. The potential of regenerative energy from these axles is not utilized by hybrid drive systems.
Further, there is little or no brake blending (e.g., proportionally braking with both the foundation brakes and the hybrid drive system) with an air brake system. This is because the hybrid drive systems are added on or retrofitted to an existing vehicle model. The system integrator does not want to de-tune or otherwise modify the air brake system that was designed to meet certain safety standards. The hybrid braking effort is always in addition to the foundation braking effort. Hence much of the available energy is lost to the foundation brakes and not recovered by the hybrid drive system. In some applications, the hybrid brake system is activated when the driver removes his/her foot from the throttle and before applying the brake. This is done in an effort to capture regenerative energy before the foundation brake is applied. The function is similar to engine or driveline retarders whereby the vehicle begins to slow down as soon as the throttle is released. Essentially there is no coasting. A typical application would be for refuse vehicles in residential service which do rapid start stop cycles. While this allows a greater portion of regenerative braking energy to go to the hybrid drive system it results in braking without the brake lights being on.
Another shortcoming of many hybrid drive systems is the need for a secondary cooling system in addition to the engine cooling system. The secondary cooling system is necessitated by the fact that the hybrid motor drive does double duty by operating at relatively high energy and power levels during the braking event and then again during the next launch and propulsion cycle. This generates appreciable heat in both modes and requires an external cooling system. The primary cooling system for the engine is typically incapable of handling this additional heat. That is, these systems operate in the temperature range of 90 to 105 degree C. which is well above the optimal 60 to 85 degree temperature range for hybrid drive systems. The secondary cooling system results in added system complexity, weight size and cost.
It would be desirable to provide a hybrid braking system which could maximize energy recovery, result in additional fuel savings and/or avoid the need for an additional cooling system.